The discovery of a new planet in our Solar System could have an unintended consequence - the elimination of Pluto in the list of planets everyone has in their heads. Is it time to wave this distant, dark piece of rock farewell?

To the casual observer, the announcement that scientists have identified a tenth planet orbiting the Sun is primarily of importance to few people other than science teachers and schoolchildren.

But, on closer examination, the revelation may have more far-reaching consequences for the way in which we think about space.

At around 3,000km across, 2003 UB313 - as it has been named - is the largest object found in our Solar System since the discovery of Neptune in 1846.

PLANETS MNEMONICS

My Very Excellent Mother Just Sent Us Nine Pizzas

My Very Earnest Mother Just Served Us Nine Pickles

Matilda Visits Every Monday, Just Stays Until Noon, Period

And it is thought to be larger than Pluto, whose status as the furthest planet from the Sun has been enshrined in accepted thought since it was identified in 1930.

And in the late 1960s scientists found that Pluto's size had been over-estimated.

There are objects which are of comparable size to Pluto, so if you think of Pluto as a planet then you should refer to those objects as planets

Prof Mark BaileyDirector of Armagh Observatory

It was first thought to be around as large as Earth, whereas accepted thought now suggests that the planet's mass is only around a fifth of the moon's.

"Today, the world knows that Pluto is not unique. There are other Plutos, just farther out in the Solar System where they are a little harder to find," says David Rabinowitz of Yale University, who was among the astronomers who discovered 2003 UB313 two years ago.

His point is echoed by Professor Mark Bailey, director of Armagh Observatory in Northern Ireland.

"Increasingly, objects are far away and there are objects which are of comparable size to Pluto, so if you think of Pluto as a planet then you should refer to those objects as planets," he says.

He estimates that there could be tens of thousands of objects beyond Neptune in the Solar System region known as the Kuiper belt, many of which may be larger than Pluto.

The discovery of 2003 UB313 comes soon after it was announced that 2003 EL61 had been found.

Planet 2003 UB313 is among several objects discovered in recent years

And a number of distant objects around the same size of Pluto have been found in recent years, including Quaoar (found in 2002) and Sedna (detected in 2004).

It is widely accepted that the struggle to provide an adequate definition of a planet is the crux of the problem.

"Originally a planet was a wandering star. Then it was something that moved across the sky. Then it was something that revolved around the Sun. The criterion about when it should be called a planet is something that is changing over time," says Prof Bailey.

"I'm sure we will continue to discover more and more objects of comparable size which will continue to challenge established thought about planets."

'Size does matter'

Dr Brian Marsden, director of the International Astronomy Union's minor planet centre, believes the simplest way to resolve the confusion is to reject Pluto's claim to being a planet on the grounds that "size does matter".

Instead he says people should accept that "we have eight planets and only an object bigger than Mars could be considered to be a planet in the future".

He argues that the disruption that would be caused to accepted thought would, ultimately, provide a more accurate understanding of space.

"School text books concentrate too much on the idea that Pluto is the ninth planet. Teaching should stress that there are hundreds of thousands of much smaller objects. Knowing a mnemonic and naming the planets is not science."

But not everyone believes science has the right, or influence to turn accepted thought on its head.

"Our culture has fully embraced the idea that Pluto is a planet and scientists have for the most part not yet realised that the term planet no longer belongs to them," says Michael Brown, one of the astronomers who discovered 2003 UB313.

Technological advances have enabled astronomers to extend their gaze

His conclusion is simple: "From now on, everyone should ignore the distracting debates of the scientists. Planets in our solar system should be defined not by some attempt at forcing a scientific definition on a thousands-of-years-old cultural term, but by simply embracing culture. Pluto is a planet because culture says it is.

"It is understandably hard for scientists to let go of a word that they think they use scientifically, but they need to."

He considers 2003 UB313 to be a planet in a "cultural" and "historical" sense, adding: "I will not argue that it is a scientific planet because there is no good scientific definition which fits our solar system and our culture and I have decided to let culture win this one.

"We scientists can continue our debates, but I hope we are generally ignored."

I would agree that Pluto is not a planet. However the Moon is so large in relation to the earth compared with other planets' moons, that if we were external observers of the Solar System, we would probably classify Earth and Moon as a binary system of satellites. Incidentally, another more relevent mnemonic of the Solar System, including the Sun, is "Space Men Vote Earth Most Jolly of the Solar Universe's Nine Planets."Simon Robinson, Birmingham, UK

I agree that the concept of many and smaller bodies circling the sun in the outer reaches of the solar system be taught, before mnemonic lists of the planets. The solar system is obviously a lot more fluid and changing than we used to think, and surely more "planetary" objects out there, increases the chance that we'll find more exo-planets, WAY out there.John, Inverness

My Very Easy Method Just Speeds Up Naming PlanetsKate, leeds

Are UB313 or Pluto planets? If Patrick Moore say they are, then they are. If he says they aren't, then they're not. QED.phil cash, lincoln

I have just been listening to the last series of 'The Hitch-hiker's Guide to the Galaxy' in which Douglas Adams predicted a 10th "Planet", which came to be known as Rupert. So is Rupert large enough to be called a planet? There have been calls for Pluto to be demoted for at least a decade, but it is stuck in the public's mind as a planet, illogical as this might be. Rupert is bigger than Pluto so, until otherwise defined, it is a planet.Mark Errington, Hereford UK

How refreshing to hear a scientist, Michael Brown, saying that scientific thought and definitions need not, and in some cases should not, determine the opinion and understanding of the wider culture. I hope he can persuade some of his more arrogant colleagues (of all disciplines) that science, however important, is just one part of our culture and should serve it, not dictate to it.Will, Southampton

Motor vehicles, every morning jam streets until neatly parked.S, hull

I think this illustrates the lag between the forefront of scientific thought and its interpretation in popular culture. In the final analysis, history has shown, cultural forces prevail over science. Not a happy conclusion.Vic Rocks, Haverhill,Suffolk,UK

My very, easy mingle jingle seems useful naming planets!Anne, london

You can't say something is right just because culture says so. Where would that kind of woolly thinking lead us? I'd much rather accept the sad fact that we've only got eight planets - not that the fact makes much difference to my daily life - than make decisions on what we feel or think is right.Brian, Liverpool

I can remember being told by a Physics Teacher many years ago that the only reason Pluto was designated a planet was to placate the Americans who had, until then, never discovered a planet. Maybe apocryphal but there may also be a grain of truth in it. What puzzles me is why we appear to have diverted from the tradition of naming the planets after Roman gods. I'm sure there are plenty left.Dave Press, Gosport

Why didn't the astrologists predict this discovery?A Sceptic, London England

How about the definition of a planet being one that a human could walk comfortably on? That would limit the size/mass to probably a bit smaller than our moon.Al, Bollington, Cheshire, UK

Ditch poor Pluto. It's been trying, unsuccessfully, to live up to the status of 'planet' on the outer fringe of our interest for years. Poor thing. Let it alone. Also - a major benefit will be the re-establishment of Holst's Planet Suite as a holistic work!Simon, Edinburgh UK

It is ridiculous to consider Pluto a planet - it's tiny size and erratic orbit clearly mark it out as one of the larger Kupier Belt objects. It is more like Sedna, Quaoar, 2003 UB61 and 2003 UB313 than it is to the outer planets like Neptune or Uranus. There are eight planets in our solar system. Scientists can be oddly sentimental at times, but they need to move on.Mike Hall, Liverpool, UK

Pluto's orbit around the Sun is wonky as well. It's too small, it isn't from our solar system. It's too different, and therefore can't count as a planet. Why not give 2003 UB313 the name Pluto and be done with it? Rhys D, Swansea, UK

Culture & science often clash. Culture used to state that the world was flat, but science has shown otherwise and culture changed to embrace the new knowledge. Either all objects above a certain size are planets or they are not. Where that cut-off should be could be debated endlessly. Other factors would obviously have some bearing, such as having an 'independent' orbit - otherwise the moon would be a planet, as would a lot of other planet's satellites.Adrian, Kendal UK

The idea that we should 'let' pluto be a planet because it is culturally accepted as such is rediculous - why don't we let the earth stay flat because it was culturally accepted once that way? A new classification that helps scientific and lay understandings of the universe should not be avoided because 'people like pluto as a planet'... The whole point of education and progress is to change what we think for the better. I'm sure people liked the old money too - but they dealt with the logical change - suck it up and deal societyJustin, London

I am an astronomer who has participated actively in this debate. For many years I was strongly against changing Pluto's status as a planet. However, the latest discoveries, especially over the last 10 years and the realization that Pluto is a tiny ball of ice smaller than 7 of the moons of other planets has really removed any credibility that it has. Even the discoverers of Pluto doubted its nature and, did not even announce it as a new planet but rather as a "Trans-Neptunian body". It's time to face facts and correct a mistake that we made 75 years ago.Mark Kidger, Tenerife, Spain